We will investigate the regulation of tyrosine synthesis in hepatoma cells growing in culture; and specifically study the origin of population density effects on the enzyme activity, the mechanism of p-chlorophenylalanine repression of the phenylalanine hydroxylase, and the transport of L-phenylalanine and L-tyrosine by the cells. The functional significance of changes in enzyme level will also be established. Normal and oncogenically transformed cells, derived from rat liver, will be used and their properties compared.